868. 筛质数

https://www.acwing.com/problem/content/submission/code_detail/6873993/

AcWing/868/868.cpp

@@ -0,0 +1,26 @@
+#include <bits/stdc++.h>
+
+using namespace std;
+
+bool isprime[1000005];
+int n, ans;
+
+int main() {
+    cin >> n;
+    for (int i = 0; i <= n; i++) {
+        isprime[i] = true;
+    }
+    isprime[0] = isprime[1] = false;
+    for (int i = 2; i <= n; i++) {
+        if (isprime[i] && 1ll * i * i <= n) {
+            for (int j = i * i; j <= n; j += i) {
+                isprime[j] = false;
+            }
+        }
+    }
+    for (int i = 1; i <= n; i++) {
+        ans += isprime[i];
+    }
+    cout << ans;
+    return 0;
+}